Taiwanese Patent No. 496133 disclosed an apparatus for controlling the inclination of a chair back, comprising: an air pressure rod disposed at the upper end of a chair leg and having an end passing through a base; and a chassis with a cushion disposed on the base. The fixed base and chassis are pivotally coupled together. A plurality of latch grooves extend downward from the rear side of the chassis. A guide base is disposed on the base and has a groove for accommodating a positioning plate. A sliding base is slidably disposed on the top surface of the guide base. A linear elongated hole and a slanted elongated hole are respectively disposed at the top surface of the guide base and sliding base. A screw passes through the elongated holes and mount the guide base and the sliding base onto the positioning plate accommodated in the groove.
An ear section of a reciprocating rod passes through one side of the sliding base, and a spring is disposed on each side of the reciprocating rod. Thus, both external sides of the spring are blocked by a blocking member at the end of the reciprocating rod. One, and one end of the reciprocating rod is fixed onto a control rod for controlling the inclination of the chair back. Therefore, when the control rod is pushed to drive the sliding base to move, and when the control rod is further pushed by a resilient prestressing force of the spring and the slanted elongated hole, the positioning plate is extended into or withdrawn from the groove to embed or withdraw the positioning plate respectively into or from the latch groove disposed at the end of the chassis for adjusting the inclination of the chair back.
However, the force of the spring driving the sliding base to move is perpendicular to the moving direction of the positioning plate and drives the positioning plate to move the elongated hole to a slanted position. As a result, the force for moving the positioning plate is only half of the force for the spring to move the sliding base. Further, the force for moving the positioning plate will be reduced drastically by the friction between the sliding base and the guide base and by the friction between the positioning plate and the groove.
Thus, the positioning plate will be easily stuck into the groove. Although the resilience of the spring can be increased to strengthen the force to push the positioning plate, it also requires users to apply a larger force to push the control rod. Therefore, a larger force is needed for operation. Since the direction of applying force to operate the control rod and the direction of the reaction of the spring have the same axial direction, the operation of the control rod will be unsmooth and uneasy, particularly when a larger force is needed to operate the control rod.